The printing system used by macOS is CUPS, short for the Common UNIX Printing System.
CUPS is a modular printing architecture that manages print queues, schedules jobs, converts documents into printer-ready formats, and sends them to local or network printers. It uses the Internet Printing Protocol, or IPP, as its main network standard, while also supporting compatibility interfaces such as the traditional lp and lpr commands.
CUPS began as a project by Michael Sweet, whose first public beta appeared in 1999. Apple adopted it as the printing system for Mac OS X 10.2 Jaguar in 2002 and later acquired the project and hired its principal developer. Apple continues to maintain its own CUPS builds for macOS.
AirPrint is different: it is Apple’s driverless printing technology for discovering and printing to compatible printers. LPR is an older print protocol and command-line interface that CUPS can support, while PDL means Printer Description Language, not the operating system’s printing system.